We found a singly charged Np(V)O 2 + complex with unprecedented kinetic inertness in aqueous solution, one million times slower than the widely accepted fast kinetics of neptunyl complexes. An inert NpO 2 + complex with a fluorescent 1,10-phenanthroline-2,9-dicarboxylate derivative was found by kinetic selection using polyacrylamide gel electrophoresis (PAGE) from a small chemical library. Autoreduction from Np(VI)O 2 2+ to Np(V)O 2 + via complexation was observed. A remarkably small spontaneous dissociation rate constant of 8 × 10 −6 s −1 (half-life of 23 h) was determined using PAGE. Selective detection of Np(V)O 2 + was achieved in PAGE with a detection limit of 68 pmol dm −3 (17 fg). This system was successfully applied to simulated radioactive waste samples. Our finding that electron-rich NpO 2 + forms a uniquely inert complex with no strong electrostatic interaction reveals a new aspect of actinide chemistry for developing a novel separation system of real radioactive material samples.